riot in firefox doesn't warn when the microphone is disabled in windows settings

### Description

There is a global windows setting:

If it is Off (like in this screenshot) Riot can't use the microphone in Firefox. I'm not sure if this is actually an upstream issue or a Riot issue, but Riot in Chrome correctly warns:

In both cases the microphone permission for the website within the browser was granted.

### Steps to reproduce

- Disable "Allow apps to access your microphone" in Windows settings
- Start a voice or video call
- (the call will initiate just like normally)
- (no audio output or input available)
- mute the microphone within Riot
- (audio output will start working, so you can hear the other call participant)

The call will initiate, but you are only able to hear the other call participant when you mute your microphone (I'd expect at least this part to work). Additionally the other side will obviously not hear you, as your microphone is disabled.

Logs being sent: yes

### Version information

- **Platform**: browser
- **Browser**: Firefox 74.0.1
- **OS**: Windows 10 (version 1909)
- **URL**: riot.im/app
- **Version**: 1.5.15